A track ball is a position control unit used in various types of digital equipment such as a computer, medical apparatus, and radar console. A ball constituting the track ball is usually a metallic spherical body whose surface is coated with a resin. The track ball converts the direction of rotation and the travel distance given to the ball by a palm or finger into a pulse signal by using two detection shafts in contact with the ball and a rotary encoder. Therefore, the property of ball rotating in contact with the two detection shafts is an important condition for maintaining the detection accuracy and operability of track ball, so that high accuracy of uniformity of resin coating thickness and balance of weight is required.
The coating of a metallic spherical body with resin in manufacturing the above-described track ball has conventionally been accomplished by heat curing of resin powder or injection molding of molten resin.
FIG. 2 is an explanatory view showing a method for coating the metallic spherical body by heat curing of resin powder. In FIG. 2, the coating of the metallic spherical body with resin is accomplished by filling powdery or pellet-form resin 21 into a hollow formed by metal molds 12A and 12B so that a metallic spherical body 10 is contained therein and by forming a resin coating heat-cured on the surface of the metallic spherical body 10 by heating the resin 21.
In this method, however, it is not easy to dispose the metallic spherical body in the center of the ball, so that the thickness of resin coating becomes nonuniform at different places, and hence the balance of weight of ball sometimes becomes bad. Also, the amount of resin capable of being used for coating treatment and the size of metallic spherical body subjected to coating treatment are limited, and hence the degree of freedom in design is limited regarding the amount of coating and the size of metallic spherical body to be coated.
FIG. 3 is an explanatory view showing a method for coating the metallic spherical body by injection molding of molten resin. In FIG. 3, the coating of the metallic spherical body with resin is accomplished by hanging the metallic spherical body 10 in a hollow formed by metal molds 13A and 13B by using string-form supports 14A and 14B and by forming a resin coating on the surface of the metallic spherical body 10 by injection molding molten resin into the hollow.
However, in this method as well, it is not easy to dispose the metallic spherical body in the center of the ball, so that the thickness of resin coating becomes nonuniform at different places, and hence the balance of weight of ball sometimes becomes bad. Also, on the surface of the metallic spherical body 10 having been subjected to coating treatment, pores are produced at locations supported by the string-form supports 14A and 14B, which not only impairs the appearance but also presents a problem in view of maintenance of detection accuracy.
An object of the present invention is to provide a metal surface coating treatment device and a surface coated metal manufacturing method capable of solving the problems with the above-described background art, providing excellent uniformity of coating thickness, good balance of weight, good appearance, and good feeling at the time of operation and contact, and having a high degree of freedom in design of the amount of coating and the size of coated metal.